1. Field of the Invention
The present invention relates to a method for manufacturing carbon nanocoils that are grown by winding carbon atoms in a helical configuration and in which the external diameter of the coil is 1000 nm or less. More specifically, the present invention relates to a method for manufacturing carbon nanocoils which utilizes an indium-tin-iron type catalyst and in which carbon nanocoils are efficiently grown on the surface of the catalyst while a hydrocarbon gas is under pyrolysis.
2. Prior Art
The growth of carbon fibers in a vapor phase while the fibers are twisted in the manner of a rope was first reported by Davis et al. (W. R. Davis, R. J. Slawson and G. R. Rigby, Nature, 171, 756 (1953)). Since the external diameter of such carbon ropes is microscopic in size, such ropes will be referred to below as carbon microcoils. Subsequently, various reports appeared concerning carbon microcoils; however, since there was a strong element of randomness involved in the production of such coils, these coils lacked reproducibility, and remained in a state that was inadequate for industrial production.
In 1990, Motojima et al. (S. Motojima, M. Kawaguchi, K. Nozaki and H. Iwanaga, Appl. Phys. Lett., 56 (1990) 321) discovered an efficient method for manufacturing carbon microcoils, and as a result of subsequent research, these researchers established a manufacturing method that showed reproducibility. In this method, a graphite substrate which is coated with a powdered Ni catalyst is placed inside a horizontal type externally heated reaction tube made of transparent quartz, and a raw-material gas is introduced perpendicularly onto the surface of the substrate from a raw-material gas introduction part located in the upper part of the reaction tube. This raw-material gas is a mixed gas of acetylene, hydrogen, nitrogen and thiophene. The exhaust gas is discharged from the bottom part of the reactiontube.
In particular, impurities such as sulfur and phosphorus, etc., are indispensable; if the amounts of these impurities are too large or too small, carbon microcoils will not grow. For example, the coil yield reaches a maximum, at a value of approximately 50%, in a case where thiophene containing sulfur is added at the rate of 0.24% relative to the total gas flow. The reaction temperature is approximately 750 to 800xc2x0 C.
The diameter of the fibers constituting such carbon microcoils is 0.01 to 1 xcexcm, the external diameter (outside diameter) of the coils is 1 to 10 xcexcm, the coil pitch is 0.01 to 1 xcexcm, and the coil length is 0.1 to 25 mm. Such carbon microcoils have a completely amorphous structure, and have superior physical properties such as electromagnetic wave absorption characteristics, etc., so that these microcoils show promise as electromagnetic wave absorbing materials.
In 1991, carbon nanotubes were discovered. Spurred by this discovery, research concerning carbon coils on the nanometer scale, i.e., carbon nanocoils, was initiated. The reason for this was that on the nanometer scale, there was a possibility that new physical properties might be discovered, so that such nanocoils showed promise as new materials in electronics and engineering, etc., in the nanometer region. However, the development of such carbon nanocoils was not easy.
In 1994, Amelinckx et al. (Amelinckx, X. B. Zhang, D. Bernaerts, X. F. Zhang, V. Ivanov and J. B. Nagy, SCIENCE, 265 (1994) 635) succeeded in producing carbon nanocoils. It was also demonstrated that while carbon microcoils are amorphous, carbon nanocoils have a graphite structure. Various types of carbon nanocoils were manufactured, and the minimum external diameter of these nanocoils was extremely small, i.e., approximately 12 nm.
The manufacturing method used by the abovementioned researchers was a method in which a metal catalyst such as Co, Fe or Ni is formed into a fine powder, the area around this catalyst is heated to a temperature of 600 to 700xc2x0 C., and an organic gas such as acetylene or benzene is caused to flow through so that this gas contacts the catalyst, thus breaking down these organic molecules. The substance produced as a result consists of carbon nanotubes with a graphite structure, and the shapes of these nanotubes are linear, curvilinear, planar spiral and coil form, etc. In other words, it will be understood that carbon nanocoils are only produced by chance, so that the coil yield is also small.
In 1999, Li et al. (W. Li, S. Xie, W. Liu, R. Zhao, Y: Zhang, W. Zhou and G. Wang, J. Material Sci., 34 (1999) 2745) succeeded again in producing carbon nanocoils. In the manufacturing method used by these researchers, a catalyst formed by covering the outer circumference of a graphite sheet with iron particles was placed in the center [sic], and the area around this catalyst was heated to 700xc2x0 C. by means of a nichrome wire. A mixed gas consisting of 10% acetylene and 90% nitrogen by volume was caused to flow through so that this gas contacted the catalyst. The flow rate of this gas was set at 1000 cc/min. The carbon nanocoils that were produced had various external diameters; the smallest nanocoils had diameters of 20 nm or 22 nm. However, this manufacturing method also showed a small coil production rate, and was extremely inadequate as an industrial production method.
Thus, a common feature of carbon coil production methods is that acetylene is used as a raw-material gas, and carbon coils are grown by means of a catalyst while pyrolyzing this acetylene. If thiophene is used as a trace gas on a Ni catalyst, a large quantity of carbon microcoils can be produced; on the other hand, if an iron catalyst is used, carbon nanocoils can be produced, although in an extremely small quantity. In other words, the development of a catalyst appears to hold the key to large-quantity production of carbon nanocoils.
Accordingly, the object of the present invention is to realize a mass production method for carbon nanocoils by developing an appropriate catalyst.
The above object is accomplished by unique steps of the present invention in a carbon nanocoil manufacturing method that manufactures carbon nanocoils which are grown by winding carbon atoms in a helical configuration and which have an external diameter of 1000 nm or less; and the unique steps of the present invention comprises:
placing an indium-tin-iron type catalyst with a desired form inside a reactor,
heating the area around the catalyst to a temperature equal to or greater than the temperature at which the hydrocarbon used as a raw material is broken down by the action of the catalyst,
causing a hydrocarbon gas to flow through the reactor so that this gas contacts the catalyst, and
allowing carbon nanocoils to grow on the surface of the catalyst while the hydrocarbon is broken down in the vicinity of the catalyst.
In the above method, the carbon nanocoils are formed by the growth of carbon nanotubes wound in a helical configuration.
Also, the indium-tin-iron type catalyst is constructed from a mixed catalyst of indium oxide and tin oxide, and a thin film of iron which is formed on the surface of this mixed catalyst.
Furthermore, the mixed catalyst consists of a thin film of a mixed catalyst of indium oxide and tin oxide formed on the surface of a glass substrate. In addition, the hydrocarbon is acetylene.
The indium-tin-iron type catalyst can be arranged (dispersed or sprayed) as a particle form inside the reactor. Moreover, the indium-tin-iron type catalyst can be arranged on the surface of a cylinder placed inside the reactor, and the cylinder can be rotated in the reactor.